Gratings and Fresnel Lenses Produced with Sol-Gel Materials

Optical communication devices typically need components that are not just physically small, but also economical. Researchers at the Korea Advanced Institute of Science and Technology in Taejon have developed a novel photosensitive sol-gel process that offers a simple method of inexpensively and accurately producing small-scale optical elements.

Byeong-Soo Bae, a professor in the department of materials science and engineering at the institute, has developed a sol-gel hybrid material that can retain features impressed upon it in the liquid phase after it is converted to a solid. Although previous hybrids have required complex lithographic and etching fabrication techniques, Bae's photosensitive material requires only a single UV exposure to pattern a desired optical element.

New material

To a traditional sol-gel hybrid, Bae adds benzyldimethylketal, a photoinitiator, and methylmethacrylic acid, a photoactive monomer. This solution is spin-coated, exposed to radiation from a 325-nm HeCd laser and baked at 150 °C for five hours. The UV exposure photopolymerizes and photolocks the active materials within the sol-gel hybrid matrix. The effect is proportional to UV intensity; regions with high intensity lock in the photoreactive compounds, while unexposed regions leave the compounds free to evaporate in the subsequent heating and drying steps. Where photoinduced reactions have taken place, the sol-gel hybrid is thicker and has a higher index of refraction.

Bae calls the new material a photohybrimer. "The photohybrimer is unique in that it undergoes a large change in both refractive index and volume," he said. "It has the potential to fabricate micro-optical devices such as gratings, holograms, microlens arrays and waveguides."

After investigating this basic process, which Bae calls photoinduced self-developing, the researchers demonstrated its utility by creating diffractive elements in this single-step process. In one proof-of-principle demonstration, they exposed the photohybrimer to an interference pattern generated by splitting the beam from the HeCd laser. They recombined the two beams at an angle with respect to each other. Because the material changes thickness and index proportionally to UV exposure, the sinusoidally varying interference patterns resulted in diffraction grating of the same period. Producing the desired grating pitch merely required the proper selection of beam recombination angle.

Optics fabrication

To further demonstrate the potential of the new material, the researchers fabricated a Fresnel lens, which consists of different diffractive zones that induce a spatially varying change in beam propagation. Although such a lens cannot bring a broadband source into crisp focus, it can focus a single wavelength well.

To produce the lens, they propagated the HeCd laser beam through a pinhole and projected the image onto the photohybrimer. The circular diffraction pattern yielded index and thickness variations that functioned as a Fresnel lens. Changing the pinhole size and projection distance alters the size of the diffraction pattern, which changes the focal length of the lens.

Bae is working with industry to fabricate waveguides from the photohybrimer and is seeking industrial partners for the mass production of other micro-optics, where he believes there is a significant advantage.

"The thermal stability and optical quality from the photohybrimer photopatterning technique is the most cost-effective, simple technique for mass production of various optical devices," he said.